The present invention relates to a direct-drive type servo valve in which a spool is directly driven by a force motor and the driving power of the force motor is balanced by the reactional force of a resilient member to stabilize the position of the spool.
Hitherto, coiled springs have been used as the resilient member for stabilizing the position of the spool in direct drive type servo motor. In the case of the coiled spring, however, it is extremely difficult to precisely coincide the center of the spool with the center of the coiled spring, because some error is inevitable in the assembling. Even if the coincidence of the center is obtained, the terminal end of the coil makes a close contact with the adjacent coil, so that the portion of the coiled spring making this close contact exhibits a higher spring constant than the portion where no contact is made. In consequence, the direction of the reactional force is inclined to the axis of the spool to generate a component of force which acts to press the spool against the wall of the sleeve. As a result, the friction between the spool and the sleeve is increased to hinder the smooth movement of the spool and to increase the hysteresis of the servo valve itself.
In order to obviate above-described problem, Japanese Patent Laid-open No. 94375 discloses a spool valve in which a diaphragm supported by the valve member is interposed between the force motor and the spool. However, it is extremely difficult to increase the spring constant while maintaining a high linearity of the characteristic between the force and the displacement. With this arrangement, therefore, it is not possible to obtain a high response characteristic of the spool valve.